Trap-integrated fluorescence detection with silicon photomultipliers for sympathetic laser cooling in a cryogenic Penning trap

We present a fluorescence-detection system for laser-cooled 9Be+ ions based on silicon photomultipliers (SiPMs) operated at 4 K and integrated into our cryogenic 1.9 T multi-Penning-trap system. Our approach enables fluorescence detection in a hermetically sealed cryogenic Penning-trap chamber with limited optical access, where state-of-the-art detection using a telescope and photomultipliers at room temperature would be extremely difficult. We characterize the properties of the SiPM in a cryocooler at 4 K, where we measure a dark count rate below 1 s-1 and a detection efficiency of 2.5(3)%. We further discuss the design of our cryogenic fluorescence-detection trap and analyze the performance of our detection system by fluorescence spectroscopy of 9Be+ ion clouds during several runs of our sympathetic laser-cooling experiment.

BASE-STEP: A transportable antiproton reservoir for fundamental interaction studies

Currently, the world's only source of low-energy antiprotons is the AD/ELENA facility located at CERN. To date, all precision measurements on single antiprotons have been conducted at this facility and provide stringent tests of fundamental interactions and their symmetries. However, magnetic field fluctuations from the facility operation limit the precision of upcoming measurements. To overcome this limitation, we have designed the transportable antiproton trap system BASE-STEP to relocate antiprotons to laboratories with a calm magnetic environment. We anticipate that the transportable antiproton trap will facilitate enhanced tests of charge, parity, and time-reversal invariance with antiprotons and provide new experimental possibilities of using transported antiprotons and other accelerator-produced exotic ions. We present here the technical design of the transportable trap system. This includes the transportable superconducting magnet, the cryogenic inlay consisting of the trap stack and detection systems, and the differential pumping section to suppress the residual gas flow into the cryogenic trap chamber.

Ultra-thin polymer foil cryogenic window for antiproton deceleration and storage

We present the design and characterization of a cryogenic window based on an ultra-thin aluminized biaxially oriented polyethylene terephthalate foil at T < 10 K, which can withstand a pressure difference larger than 1 bar at a leak rate <1×10-9 mbar l/s. Its thickness of ∼1.7 μm makes it transparent to various types of particles over a broad energy range. To optimize the transfer of 100 keV antiprotons through the window, we tested the degrading properties of different aluminum coated polymer foils of thicknesses between 900 and 2160 nm, concluding that 1760 nm foil decelerates antiprotons to an average energy of 5 keV. We have also explicitly studied the permeation as a function of coating thickness and temperature and have performed extensive thermal and mechanical endurance and stress tests. Our final design integrated into the experiment has an effective open surface consisting of seven holes with a diameter of 1 mm and will transmit up to 2.5% of the injected 100 keV antiproton beam delivered by the Antiproton Decelerator and Extra Low ENergy Antiproton ring facility of CERN.

BASE-high-precision comparisons of the fundamental properties of protons and antiprotons

Abstract

The BASE collaboration at the antiproton decelerator/ELENA facility of CERN compares the fundamental properties of protons and antiprotons with ultra-high precision. Using advanced Penning trap systems, we have measured the proton and antiproton magnetic moments with fractional uncertainties of 300 parts in a trillion (p.p.t.) and 1.5 parts in a billion (p.p.b.), respectively. The combined measurements improve the resolution of the previous best test in that sector by more than a factor of 3000. Very recently, we have compared the antiproton/proton charge-to-mass ratios with a fractional precision of 16 p.p.t., which improved the previous best measurement by a factor of 4.3. These results allowed us also to perform a differential matter/antimatter clock comparison test to limits better than 3 %. Our measurements enable us to set limits on 22 coefficients of CPT- and Lorentz-violating standard model extensions (SME) and to search for potentially asymmetric interactions between antimatter and dark matter. In this article, we review some of the recent achievements and outline recent progress towards a planned improved measurement of the antiproton magnetic moment with an at least tenfold improved fractional accuracy.

Graphic Abstract

A high-Q superconducting toroidal medium frequency detection system with a capacitively adjustable frequency range >180 kHz

We describe a newly developed polytetrafluoroethylene/copper capacitor driven by a cryogenic piezoelectric slip-stick stage and demonstrate with the chosen layout cryogenic capacitance tuning of ≈60 pF at ≈10 pF background capacitance. Connected to a highly sensitive superconducting toroidal LC circuit, we demonstrate tuning of the resonant frequency between 345 and 685 kHz, at quality factors Q > 100 000. Connected to a cryogenic ultra low noise amplifier, a frequency tuning range between 520 and 710 kHz is reached, while quality factors Q > 86 000 are achieved. This new device can be used as a versatile image current detector in high-precision Penning-trap experiments or as an LC-circuit-based haloscope detector to search for the conversion of axion-like dark matter to radio-frequency photons. This new development increases the sensitive detection bandwidth of our axion haloscope by a factor of ≈1000.

A 16-parts-per-trillion measurement of the antiproton-to-proton charge-mass ratio

The standard model of particle physics is both incredibly successful and glaringly incomplete. Among the questions left open is the striking imbalance of matter and antimatter in the observable universe¹, which inspires experiments to compare the fundamental properties of matter/antimatter conjugates with high precision^2-5. Our experiments deal with direct investigations of the fundamental properties of protons and antiprotons, performing spectroscopy in advanced cryogenic Penning trap systems⁶. For instance, we previously compared the proton/antiproton magnetic moments with 1.5 parts per billion fractional precision^7,8, which improved upon previous best measurements⁹ by a factor of greater than 3,000. Here we report on a new comparison of the proton/antiproton charge-to-mass ratios with a fractional uncertainty of 16 parts per trillion. Our result is based on the combination of four independent long-term studies, recorded in a total time span of 1.5 years. We use different measurement methods and experimental set-ups incorporating different systematic effects. The final result, [Formula: see text], is consistent with the fundamental charge-parity-time reversal invariance, and improves the precision of our previous best measurement⁶ by a factor of 4.3. The measurement tests the standard model at an energy scale of 1.96 × 10^-27 gigaelectronvolts (confidence level 0.68), and improves ten coefficients of the standard model extension¹⁰. Our cyclotron clock study also constrains hypothetical interactions mediating violations of the clock weak equivalence principle (WEP_cc) for antimatter to less than 1.8 × 10^-7, and enables the first differential test of the WEP_cc using antiprotons¹¹. From this interpretation we constrain the differential WEP_cc-violating coefficient to less than 0.030.

Sympathetic cooling of a trapped proton mediated by an LC circuit

Efficient cooling of trapped charged particles is essential to many fundamental physics experiments1,2, to high-precision metrology3,4 and to quantum technology5,6. Until now, sympathetic cooling has required close-range Coulomb interactions7,8, but there has been a sustained desire to bring laser-cooling techniques to particles in macroscopically separated traps5,9,10, extending quantum control techniques to previously inaccessible particles such as highly charged ions, molecular ions and antimatter. Here we demonstrate sympathetic cooling of a single proton using laser-cooled Be+ ions in spatially separated Penning traps. The traps are connected by a superconducting LC circuit that enables energy exchange over a distance of 9 cm. We also demonstrate the cooling of a resonant mode of a macroscopic LC circuit with laser-cooled ions and sympathetic cooling of an individually trapped proton, reaching temperatures far below the environmental temperature. Notably, as this technique uses only image-current interactions, it can be easily applied to an experiment with antiprotons1, facilitating improved precision in matter-antimatter comparisons11 and dark matter searches12,13.

An Anthropomorphic Soft Exosuit for Hand Rehabilitation

Functional impairment of the hand, for example after a stroke, can be partially improved by intensive training. This is currently done by physiotherapy and the optimal intensity of hand rehabilitation programs is usually not reached due to a lack in human resources (high costs) and patients fatigue. In this work a cost-effective soft exosuit to support the hand's grasping function is presented. The system is based on tendon-like wires and all fingers except the little finger are actuated. Each of the remaining four fingers is bidirectionally controlled by an electrical motor. This allows a variety of gripping situations, e.g. a power or precision grip. Our prototype weighs 435g, including the battery and can be worn on the upper arm. The force applicable for a power grip exceeds 20N with a maximum gripping frequency of 4Hz. Furthermore, a force control is implemented, giving the wearer the opportunity to grab sensitive objects. All components used are available in different sizes, allowing a quick and individual preparation per patient. Therefore, our prototype can be used for rehabilitation while doing activities of daily living (ADL) starting on the day of the injury.

Dawning of the N=32 Shell Closure Seen through Precision Mass Measurements of Neutron-Rich Titanium Isotopes

A precision mass investigation of the neutron-rich titanium isotopes ^{51-55}Ti was performed at TRIUMF's Ion Trap for Atomic and Nuclear science (TITAN). The range of the measurements covers the N=32 shell closure, and the overall uncertainties of the ^{52-55}Ti mass values were significantly reduced. Our results conclusively establish the existence of the weak shell effect at N=32, narrowing down the abrupt onset of this shell closure. Our data were compared with state-of-the-art ab initio shell model calculations which, despite very successfully describing where the N=32 shell gap is strong, overpredict its strength and extent in titanium and heavier isotones. These measurements also represent the first scientific results of TITAN using the newly commissioned multiple-reflection time-of-flight mass spectrometer, substantiated by independent measurements from TITAN's Penning trap mass spectrometer.

mRNA-Mediated Gene Supplementation of Toll-Like Receptors as Treatment Strategy for Asthma In Vivo

Asthma is the most common chronic disease in childhood. Although several therapeutic options are currently available to control the symptoms, many drugs have significant side effects and asthma remains an incurable disease. Microbial exposure in early life reduces the risk of asthma and several studies have suggested protective effects of Toll-like receptor (TLR) activation. We showed previously that modified mRNA provides a safe and efficient therapeutic tool for in vivo gene supplementation. Since current asthma drugs do not take patient specific immune and TLR backgrounds into consideration, treatment with tailored mRNA could be an attractive approach to account for the patient's individual asthma phenotype. Therefore, we investigated the effect of a preventative treatment with combinations of Tlr1, Tlr2 and Tlr6 mRNA in a House Dust Mite-induced mouse model of asthma. We used chemically modified mRNA which is-in contrast to conventional viral vectors-non-integrating and highly efficient in gene transfer. In our study, we found that treatment with either Tlr1/2 mRNA or Tlr2/6 mRNA, but not Tlr2 mRNA alone, resulted in better lung function as well as reduced airway inflammation in vivo. The present results point to a potentially protective effect of TLR heterodimers in asthma pathogenesis.

Expression of urokinase-type plasminogen-activator (upa) and its receptor (upar) in human ovarian-cancer cells and in-vitro invasion capacity

Expression of urokinase-type plasminogen activator (uPA), tissue-type plasminogen activator (t-PA), their inhibitor PAI-1 and the uPA-receptor (uPAR) was characterized in six human tumor cell lines (OV-MZ-6, -10, -13, -15, -19 and OVCAR-3) established from patients with cystadenocarcinoma of the ovary. The invasive potential of the ovarian cancer cell lines determined in an in vitro invasion assay did neither correlate with the antigen level of uPA, t-PA, PAI-1 or uPAR nor with the cell surface uPA activity, however, did correlate with the cell surface-bound plasmin activity. The in vitro invasiveness of three cancer cell lines selected displaying a different pattern of uPA and uPAR expression was significantly inhibited by a recombinant soluble truncated form of the uPAR functioning as a scavenger for uPA. Our results suggest that the interference of the uPA/uPAR interaction leads to a reduced in vitro invasiveness of human ovarian cancer cells independent of the level of uPA and uPAR expression.

[Comparative studies of sinus floor elevation with autologous or allogeneic bone tissue]

In 63 patients, 82 elevations of the maxillary sinus were performed. As augmentation, materials autografts from the iliac crest (combined with alveolar ridge augmentations in 16 sinus lifts) were transplanted in 39 cases and osteoinductive, allogeneic bone powder (AAA bone (autolyzed, antigen-extracted, allogeneic bone): n = 8, DFDBA (demineralized freeze-dried bone allograft) and/or Grafton (demineralized bone matrix gel): n = 35) were used in 43 cases. Some 4-6 months after implantation, osteoinductive, allogeneic (demineralized) bone implants showed radio-opaque areas as an equivalent of bone formation. Histological examinations revealed that osteoinductive implants were completely transformed into patients' own bone tissue. The average augmentation height after autograft transplantations was 14 (+/- 3) mm in comparison with 9 (+/- 3) mm after allograft implantations. Histologically as well as radiologically no differences of the bone quality could be determined between the two augmentation materials. Endoscopic controls showed, in both groups, nonirritated mucous membranes. On an average 2 endosseous implants (Bone Lock or ITI-screw implants) were inserted into the augmentated maxillary sinus floors in both groups. No osseointegration was achieved in 4 out of 67 dental implants when bone autografts were used and in 2 out of 74 dental implants of the allogeneic bone group. Patients with bone autografts suffered from postoperative complaints on an average of 19 (+/- 9) days (without consideration of 2 patients with postoperative complaints persisting for more than 90 days). The average postoperative complaints of recipients of allogeneic bone implants continued for 3 (+/- 5) days. The 13 patients who underwent an ambulant sinus lift procedure with allogeneic bone powder were already symptom-free several hours after the operation. Under critical consideration of all investigated parameters, osteoinductive bone implants are preferable to iliac bone autografts for maxillary sinus augmentations in those cases in which no additional alveolar ridge augmentation is required.

U1 snRNP-ASF/SF2 interaction and 5' splice site recognition: characterization of required elements

Members of the SR family of proteins, can collaborate with U1 snRNP in the recognition of 5' splice sites in pre-messenger RNAs. We have previously shown that purified U1 snRNP and ASF/SF2 form a ternary complex with pre-mRNA, which is dependent on a functional 5' splice site. In this manuscript we dissect the requirements for the formation of this complex. Sequences in the pre-mRNA, domains in ASF/SF2 and components of the U1 snRNP particle are shown to be required for complex formation. We had shown that sequences at the 5' splice site of PIP7. A are necessary and now we show these are sufficient for complex formation. Furthermore, we show that one functional RNA binding domain and the RS domain are both required for ASF/SF2 to participate in complex formation. The RNA binding domains were redundant in this assay, suggesting that either domain can interact with the pre-messenger RNA. Finally, our experiments show no function for the U1-specific A protein in complex formation, whereas a function for U1-specific C protein was strongly suggested. The study of the earliest interactions between pre-mRNA and splicing factors suggests a model for 5' splice site recognition.

[Is dopaminergic therapy immunologically rejuvinating? Increased interferon-gamma production with the dopaminergic agent lisuride]

Eighteen patients with advanced Parkinson's disease (n = 13) or dopamine-sensitive dystonia (n = 5) were treated with the dopaminergic agent, lisuride, applied as a long-term subcutaneous infusion. The results were compared with those obtained in a group of younger, and a group of older, healthy volunteers. The liberation of gamma-interferon (gamma-IFN) following mitogenic stimulation of whole blood with phytohemagglutinin (PHA) was highly significantly elevated in comparison with the group of older healthy volunteers, and clearly, but not significantly, elevated in comparison with the younger group. There was no difference between patients with dystonia and those with Parkinson's disease. The effect observed is thus probably due to lisuride. This effect might explain the longer life expectancy and reduced proclivity for infection shown by patients with Parkinson's disease. It needs to be determined whether, on the basis of these initial data, a therapeutic principle for the treatment of diseases that can be directly influenced by gamma-IFN can be derived.

Marburg virus gene 4 encodes the virion membrane protein, a type I transmembrane glycoprotein

Gene 4 of Marburg virus, strain Musoke, was subjected to nucleotide sequence analysis. It is 2,844 nucleotides long and extends from genome position 5821 to position 8665 (EMBL Data Library, emnew: MVREPCYC [accession no. Z12132]). The gene is flanked by transcriptional signal sequences (start signal, 3'-UACUUCUUGUAAUU-5'; termination signal, 3'-UAAUUCUUUUU-5') which are conserved in all Marburg virus genes. The major open reading frame encodes a polypeptide of 681 amino acids (M(r), 74,797). After in vitro transcription and translation, as well as expression in Escherichia coli, this protein was identified by its immunoreactivity with specific antisera as the unglycosylated form of the viral membrane glycoprotein (GP). The GP is characterized by the following four different domains: (i) a hydrophobic signal peptide at the amino terminus (1 to 18), (ii) a predominantly hydrophilic external domain (19 to 643), (iii) a hydrophobic transmembrane anchor (644 to 673), and (iv) a small hydrophilic cytoplasmic tail at the carboxy terminus (674 to 681). Amino acid analysis indicated that the signal peptide is removed from the mature GP. The GP therefore has the structural features of a type I transmembrane glycoprotein. The external domain of the protein has 19 N-glycosylation sites and several clusters of hydroxyamino acids and proline residues that are likely to be the attachment sites for about 30 O-glycosidic carbohydrate chains. The region extending from positions 585 to 610 shows significant homology to a domain observed in the envelope proteins of several retroviruses and Ebola virus that has been suspected to be responsible for immunosuppressive properties of these viruses. A second open reading frame of gene 4 has the coding capacity for an unidentified polypeptide 112 amino acids long.

Carbohydrate structure of Marburg virus glycoprotein

Marburg virus was propagated in E6 cells, a cloned cell line of Vero cells, in the presence of [6-3H]glucosamine. Radiolabelled viral glycoprotein was digested with trypsin, and oligosaccharides were liberated by sequential treatment with endo-beta-N-acetylglucosaminidase H, peptide-N4-(N-acetyl-beta-glucosaminyl)asparagine amidase F and O-glycosidase, by beta-elimination, and by alkaline hydrolysis. After fractionation by HPLC and gel filtration, glycans were characterized chromatographically, by digestion with exoglycosidases and, in part, by methylation analysis and liquid secondary ion mass spectrometry. The oligosaccharide structures thus established include oligomannosidic and hybrid-type N-glycans, as well as neutral fucosylated bi-, tri- and tetraantennary species, most of which carry an additional bisecting N-acetylglucosamine. In addition, high amounts of neutral mucin-type O-glycans with type-1 and type-2 core structures were detected. None of the glycans present in this viral glycoprotein carried sialic acid residues.

Marburg virus, a filovirus: messenger RNAs, gene order, and regulatory elements of the replication cycle

The genome of Marburg virus (MBG), a filovirus, is 19.1 kb in length and thus the largest one found with negative-strand RNA viruses. The gene order - 3' untranslated region-NP-VP35-VP40-GP-VP30-VP24-L-5' untranslated region-resembles that of other non-segmented negative-strand (NNS) RNA viruses. Six species of polyadenylated subgenomic RNAs, isolated from MBG-infected cells, are complementary to the negative-strand RNA genome. They can be translated in vitro into the known structural proteins NP, GP (non-glycosylated form), VP40, VP35, VP30 and VP24. At the gene boundaries conserved transcriptional start (3'-NNCUNCNUNUAAUU-5') and stop signals (3'-UAAUUCUUUUU-5') are located containing the highly conserved pentamer 3'-UAAUU-5'. Comparison with other NNS RNA viruses shows conservation primarily in the termination signals, whereas the start signals are more variable. The intergenic regions vary in length and nucleotide composition. All genes have relatively long 3' and 5' end non-coding regions. The putative 3' and 5' leader RNA sequences of the MBG genome resemble those of other NNS RNA viruses in length, conservation at the 3' and 5' ends, and in being complementary at their extremities. The data support the concept of a common taxonomic order Mononegavirales comprising the Filoviridae, Paramyxoviridae, and Rhabdoviridae families.

The nucleotide sequence of the L gene of Marburg virus, a filovirus: homologies with paramyxoviruses and rhabdoviruses

The nucleotide sequence of the L gene of Marburg virus, strain Musoke, has been determined. The L gene has a single long open reading frame encoding a polypeptide of 2330 amino acids (MW 267,175) that represents the viral RNA-dependent RNA polymerase. The putative transcription start signal (3'CUACCUAUAAUU 5') and the termination signal (3' UAAUUCUUUUU 5') of the gene could be identified. Computer-assisted comparison of the L protein with L proteins of other nonsegmented negative-stranded RNA viruses (Paramyxoviridae: Sendai virus, Newcastle disease virus, human parainfluenza 3 virus, measles virus, human respiratory syncytial virus; Rhabdoviridae: vesicular stomatitis virus, rabies virus) revealed significant homologies primarily in the N-terminal half of the proteins. We have identified three common conserved boxes (A, B, and C) among filo-, paramyxo-, and rhabdovirus L proteins, which are probably involved in the polymerase function. The L proteins can be divided into an N-terminal half, which seems to accommodate the common enzymatic sites, and a C-terminal half carrying virus specific peculiarities. The data presented here suggest a common evolutionary history for all nonsegmented negative-stranded RNA viruses and show that filoviruses are more closely related to paramyxo- than to rhabdoviruses.

Evidence for occurrence of filovirus antibodies in humans and imported monkeys: do subclinical filovirus infections occur worldwide?

In the present serological study 120 monkey sera from different species originating from the Philippines, China, Uganda and undetermined sources and several groups of human sera comprising a total of 1288 specimens from people living in Germany were examined for the presence of antibodies directed against filoviruses (Marburg virus, strain Musoke/Ebola virus, subtype Zaire, strain Mayinga/Reston virus). Sera were screened using a filovirus-specific enzyme-linked immunosorbent assay (ELISA). ELISA-positive sera were then confirmed by the indirect immunofluorescence technique, Western blot technique, and a blocking assay, and declared positive when at least one confirmation test was reactive. Altogether 43.3% of the monkey sera and 6.9% of the human sera reacted positively with at least one of the three different filovirus antigens. The blocking assays show that antibodies, detected in the sera, are directed to specific filovirus antigens and not caused by antigenic cross-reactivity with hitherto unknown agents. Data presented in this report suggest that subclinical filovirus infections may also occur in humans and in subhuman primates. They further suggest that filoviruses are not restricted to the African continent.

Structure and assembly of hemagglutinin mutants of fowl plague virus with impaired surface transport

Five temperature-sensitive mutants of influenza virus A/FPV/Rostock/34 (H7N1), ts206, ts293, ts478, ts482, and ts651, displaying correct hemagglutinin (HA) insertion into the apical plasma membrane of MDCK cells at the permissive temperature but defective transport to the cell surface at the restrictive temperature, have been investigated. Nucleotide sequence analysis of the HA gene of the mutants and their revertants demonstrated that with each mutant a single amino acid change is responsible for the transport block. The amino acid substitutions were compared with those of mutants ts1 and ts227, which have been analyzed previously (W. Schuy, C. Will, K. Kuroda, C. Scholtissek, W. Garten, and H.-D. Klenk, EMBO J. 5:2831-2836, 1986). With the exception of ts206, the changed amino acids of all mutants and revertants accumulate in three distinct areas of the three-dimensional HA model: (i) at the tip of the 80-A (8-nm)-long alpha helix, (ii) at the connection between the globular region and stem, and (iii) in the basal domain of the stem. The concept that these areas are critical for HA assembly and hence for transport is supported by the finding that the mutants that are unable to leave the endoplasmic reticulum at the nonpermissive temperature do not correctly trimerize. Upon analysis by density gradient centrifugation, cross-linking, and digestion with trypsin and endoglucosaminidase H, two groups can be discriminated among these mutants: with ts1, ts227, and ts478, the HA forms large irreversible aggregates, whereas with ts206 and ts293, it is retained in the monomeric form in the endoplasmic reticulum. With a third group, comprising mutants ts482 and ts651 that enter the Golgi apparatus, trimerization was not impaired.

[Value of computer tomography in staging urological tumors before radical surgery]

The authors investigated by conventional CT 66 urological tumorous patients before radical operations, and compared with the findings of the histological investigations of the lymph nodes removed by the lymphadenectomy. The coincidence between the two investigations were 80%.

Glycosylation and oligomerization of the spike protein of Marburg virus

The oligosaccharide side chains of the glycoprotein of Marburg virus (MW 170,000) have been analyzed by determining their sensitivity to enzymatic degradation and their reactivity with lectins. It was found that they consist of N- and O-glycans. Studies employing chemical cross-linking showed that the glycoprotein is present as a homotrimer in the viral envelope.

[Malignant schwannoma of the renal capsule]

The authors report on a malignant schwannoma originating in the capsule of the right kidney. By sonography, nephroangiography, cavography, computer tomography and bone scanning, metastases in the kidney or a retroperitoneal tumor could be diagnosed. After transperitoneal exploration, the right kidney and mesenteric metastases were removed. Due to tumor infiltration into the liver and tumor masses in the retroperitoneum, we confined our operation to nephrectomy and palliative excision of retroperitoneal metastases. The patient developed pulmonary metastases postoperatively and died three months after the operation.

Mutations blocking the transport of the influenza virus hemagglutinin between the rough endoplasmic reticulum and the Golgi apparatus

Mutants ts1 and ts227 of fowl plague virus have a temperature-sensitive defect in the transport of the hemagglutinin from the rough endoplasmic reticulum to the Golgi apparatus. The primary structure of the hemagglutinin of the mutants and of a number of revertants derived from them has been analysed by nucleotide sequencing. The transport block of the hemagglutinin of ts227 can be attributed to a single amino acid exchange. It involves the replacement of aspartic acid at position 457 by asparagine thereby introducing a new glycosylation site which appears to be located in a cryptic position in the lower part of the hemagglutinin stalk. Attachment of carbohydrate to this site is temperature-dependent. At permissive temperature only a small fraction of the monomers (approximately 30%) is glycosylated in this position, whereas at nonpermissive temperature this is the case with all subunits. The data suggest that under the latter conditions the new oligosaccharide interferes by steric hindrance with the trimerization of the hemagglutinin. The hemagglutinin of ts1 has an essential amino acid exchange at position 275 where serine is replaced by glycine. This substitution may increase the flexibility of the molecule in the hinge region between the globular domain and the stalk. The exchange of a conserved glutamic acid residue at position 398 that is involved in the interaction between different monomers contributes also to the structural instability of the ts1 hemagglutinin. These observations support the notion that the transport of the hemagglutinin from the rough endoplasmic reticulum to the Golgi apparatus depends on trimer assembly.